1. Field of the Invention
The present invention relates to a fabrication method for a semiconductor device, and more specifically, to a method for forming a well photoresist pattern having a rounded upper edge. The rounded upper edge in the photoresist pattern may result from defocusing the depth of focus (DOF) of light at a photo exposure step.
2. Description of the Related Art
Photolithography, a basic technique for making a highly integrated semiconductor device, is a process for forming a photoresist pattern on a semiconductor substrate using light.
A conventional method for forming a photoresist pattern is as follows. When a substrate having a photoresist is exposed to light through a mask having a pattern, a photochemical reaction is performed. The light transmitted to the substrate should have a best focus (0 focus) by adjusting the depth of focus (DOF) of the light. In one such process, after eliminating the photo-exposed photoresist using a developing solution, only the photoresist pattern to form a device is left on the substrate. Here, the photoresist pattern has a substantially perpendicular-shaped upper edge. This photoresist pattern plays a mask role on subsequent processes, such as etching and/or ion implantation.
FIG. 1 shows a conventional well ion-implantation process using a photoresist pattern.
A well ion-implantation process is performed on a semiconductor substrate 10 having a photoresist pattern 30a. Here, some of ions 60 injected into the substrate 10 may be scattered as a result of interaction with the perpendicular edge of the photoresist pattern 30a. If scattered ions 60a enter an active area 70, a well proximity effect happens. More specifically, a gate electrode 70a and a source/drain region 70b are formed in the active area 70 by the following process. Therefore, if the scattered ions 60a enter an active area 70, the device may have an increased threshold voltage and a decreased saturation current.